The present invention relates to the construction of electrostatic precipitators, and more particularly to a conductive and corrosion-resistant material for the collecting electrodes and other parts of such precipitators.
Electrostatic precipitators consist of collecting electrodes, which may be tubes, plates or other suitable members, with associated high-voltage or corona discharge electrodes to maintain a strong electric field which places a charge on particles passing through it. The charge particles deposit on the adjacent collecting electrodes from which they may be removed in any suitable manner. In many cases, the collecting electrodes take the form of vertical tubes with a high-voltage electrode or wire extending centrally through each tube. Air or gas carrying particulate matter, or other impurities to be removed, passes through the tubes where the particles are charged and deposit on the inner surfaces of the tubes which are grounded or otherwise maintained at a sufficient potential difference from the high-voltage electrodes.
Precipitators of this type are frequently used for removing corrosive materials such as acid mists from air or other gas streams. The collecting electrodes and the enclosing casing must, therefore, possess a high degree of corrosion resistance and the electrodes must have good electrical conductivity. In conventional practice, such collecting electrode tubes and the casing walls have usually been made of, or lined with, lead which has good corrosion resistance to most acids, although it is not a desirable structural material, since it is soft and mechanically weak. The use of lead collecting electrodes and casing walls, therefore, has been common practice because nothing better was available. As an alternative to lead, it has been proposed to utilize fiberglass reinforced resins for the collecting electrodes of precipitators intended for this type of service. Such glass reinforced resins are good structural materials as they have high mechanical strength and can readily be fabricated into any desired sizes and shapes. This type of material, however, is an electrical insulator, and is not always sufficiently corrosion resistant. Fiberglass reinforced resin can be made conductive by mixing a sufficient quantity of graphite powder in at least the surface layers of the resin to give it some conductivity, and it has also been proposed to utilize thin graphite veils in place of the fiberglass reinforcement in the surface layer of the material. Both of these expedients provide some conductivity, but can cause fabrication difficulties and the specific resistivity and surface resistance are still undesirably high, which can cause high voltage gradients over the surface of the material and destructive sparking. The disadvantages of this type of material, therefore, have discouraged its use in electrostatic precipitators.